1. Field of the Invention
The present invention relates to a motor vehicle automatic transmission and more particularly to a control of a motor vehicle automatic transmission. More specifically, the present invention pertains to a control of an automatic transmission having a hydraulic torque converter and a multiple-stage transmission gear mechanism.
2. Description of Prior Art
Conventional automatic transmissions for motor vehicles include a torque converter and a multiple-stage transmission gear mechanism connected with the torque converter to be driven thereby. The gear mechanism generally has three forward drive stages and one reverse drive stage. In recent automatic transmissions such as the one disclosed by the published U.K. patent application No. 2,057,602, there is arranged an overdrive gear mechanism between the torque converter and the transmission gear mechanism so that an overdrive gear stage is additionally provided. The transmission is provided with a hydraulic control system for selecting an appropriate one of the gear stages in accordance with the vehicle operating conditions, such as the engine output and the vehicle speed.
As one of control modes performed by the control system, there is provided a kick-down control wherein the gear mechanism is shifted down to a lower gear stage when an engine control member such as an engine throttle valve actuating member is moved to a substantially full power position. In an automatic transmission having such kickdown control mode, there is a possibility that the gear mechanism be shifted down from the highest gear stage by two stages when the engine control member is actuated for a kick down control. Such two stage shift down is not preferable because there will be produced an uncomfortable shock. Japanese patent publication No. 56-53137 discloses a vehicle automatic transmission control system in which a two stage shift down is prevented. According to the control system proposed by the Japanese publication, when the manual shift lever is moved from the "D" position to the "2" position with the gear mechanism in the fourth or overdrive stage, the gear mechanism is at first shifted down to the third stage and then to the second stage. It should however be noted that the proposed control system deals only with the shift down control under an operation of the manual control lever so that it does not solve the problem of shift down shock under a kick down control.